1. Field of the Invention
This invention relates to an injection molding apparatus having a unique plasticizing structure capable of constantly maintaining molding resin in its uniformly plasticized state, thereby to produce molded articles of high quality.
2. Description of the Prior Art
There has been so far widely used an in-line screw type injection molding apparatus having a rotating screw movable back and forth in a heating cylinder. Molding material supplied into a plasticizing space defined between the inner surface of the heating cylinder and the circumferential surface of the screw is plasticized with not only the heat essentially generated by the heating cylinder but also shearing calorific stress caused by the rotation of the screw. While feeding out the plasticized material toward the cylinder head, the rotating screw is forced backward by the reaction of the molding material. Then, the screw stops rotating and thrusts forward to inject the plasticized molding material from the cylinder into a mold.
In the conventional injection molding apparatus of this type, the plasticizing space between the inner heating surface of the heating cylinder and the outer surface of the screw is formed in the shape of a thin tube extending parallel with the axis of the cylinder.
In the conventional molding apparatus, the tubular plasticizing space formed in the heating cylinder and extending parallel with the axis of the cylinder entails various problems as described below.
The unit constituted by the heating cylinder and screw in the conventional molding apparatus necessitates a rather long plasticizing passage for the molding material in order to uniformly plasticize the molding material and allow the screw to sufficiently move back and forth in the cylinder. Thus, the conventional molding apparatus is inevitably complicated in structure and made large in size. In particular, as the screw having a feed zone, plasticization zone, and metering zone is necessarily made long, the heating cylinder and injection mechanism in the conventional apparatus become long. As one example, the injection mechanism capable of exerting a mold clamping force of 25 tons has generally a length of about 350 mm to 400 mm, and that capable of exerting a molding clamping force of 200 tons becomes as long as about 1000 mm to 1500 mm in length.
Furthermore, the plasticizing space between the heating cylinder and the screw must be narrowed down to less than 0.02 mm in general in order to allow the screw to move back and forth without degrading plasticizing efficiency. However, the work of inserting the screw into the heating cylinder having an inner diameter slightly larger than the outer diameter of the screw to assemble the molding apparatus turns out to be very onerous. In addition, the metallic heating cylinder and metallic screw which are in close contact with each other is worn out during the course of prolonged service.
Moreover, the screw is disadvantageous in that the plasticizing space is fixedly defined between the cylinder and thus, it cannot be adjusted in accordance with the quality or properties such as viscosity of the molding resin. As a result, the molding resin containing fiberglass or other specific composite resin is rather restricted in size of annexations such as fiberglass and other conditions. When such resin containing fiberglass is continuously dealt with, the screw is easily worn out by as much as about 0.03 mm to 0.15 mm, consequently degrading the plasticizing efficiency.
In addition, since the plasticizing space formed in a tube extends parallel to the rotation axis of the screw, the screw has to stop rotating to cease the plasticization of the molding material, with the result that the molding material to be injected cannot be uniformly plasticized nor injected with a high efficiency.
Besides, since the conventional molding apparatus is generally provided with a check valve near a nozzle in the cylinder head, the injection of the molten resin cannot be stably performed due to different injecting conditions and disturbance. Therefore, in order to perform proper injection molding for obtaining a precise molded article by utilization of the conventional molding apparatus, the injection molding operation has had to be executed while keeping the molding conditions under observation in such a manner that an excessive amount of molding material is constantly supplied so that the molding material can be added at all times in accordance with the cushioning action brought about by the molding material being injected from the plasticizing space into the mold. Thus, supplementary molding material is complementarily supplied according to the amount of the molding material in the plasticizing space, as a result of which a molded article is formed of molding resins different in viscosity, thus causing unevenness on moldings and irregularities in injection shots.
In addition to the aforementioned in-line screw type molding apparatus, there have been conventionally known a plunger type injection molding apparatus using a plunger for injecting molding material heated by a torpedo, a plunger preplasticating type injection molding apparatus having a multi-cylinder structure formed by combining the plunger type and in-line screw type molding apparatuses, and a screw preplasticating type injection molding apparatus. However, these conventional molding apparatuses have a common disadvantage of being inefficient in plasticizing the molding material and complicated in structure, and therefore, cannot receive general application.